The present invention relates to an active matrix type LCD (Liquid Crystal Display) using thin film transistors as switching devices, and a method of producing the same and, more particularly, to an active matrix LCD capable of lowering wiring resistance without increasing the number of photolithography steps, and a method of producing the same.
Thin film transistors implemented by amorphous or polycrystalline silicon, CdSe or similar semiconductor are attracting increasing attention as switching devices for use in an active matrix LCD. An LCD using such thin film transistors as switching devices is disclosed in, e.g., Japanese Patent Publication No. 2501411 corresponding to Japanese Patent Laid-Open Publication No. 6-160906. However, a wide screen or high definition is not achievable with the LCD taught in the above document for the following reason. Because a pixel electrode and a drain bus line are formed by a single photolithographic step, the drain bus line must be implemented by a transparent conductive film whose resistance is several ten times as high as the resistance of a metal film, e.g., ITO (Indium Tin Oxide) having about twenty times higher volume resistivity in terms of ITO/Cr. While this problem may be solved if a low resistance metal film is provided above or below the rain bus line, this kind of scheme is not practicable without resorting to one or more photolithographic steps. An increase in the number of photolithographic steps directly translates not only into an increase in the number of indirect members and an increase in the number of times of use of apparatuses, but also results in a decrease in yield which would noticeably increase the cost. It is therefore necessary to lower wiring resistance without increasing the number of photolithographic steps.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 57-85247, 1-179345, and 6-160906.